The present invention relates to the determination of the concentration of substances of interest in a liquid, by stripping analysis. More particularly, the invention relates to a method of stripping analysis where ultrasound is conducted on the working electrode.
Stripping analysis is carried out in a measurement cell having three electrodes, namely a working electrode, a counter electrode and a reference electrode. The three electrodes contact a sample of a liquid under test (which may be in the form of a slurry). The potential difference between the working electrode and the reference electrode is controlled and the current that flows through the working electrode, due to oxidation/reduction reactions of the ions in the liquid under test, is measured. The potentials at which current peaks are observed are characteristic of the substances that are present in the liquid under test, and the size of a current peak indicates the concentration of the particular substance in the liquid under test.
Stripping analysis is generally used for measuring the concentration of anions and cations of substances of interest in a liquid. In most cases the working electrode is a hanging mercury drop electrode or a mercury film electrode. These are best suited for analyzing mercury-soluble metals. Measurements of other metals, such as iron, nickel and cobalt, are often carried out by means of electrodes made of solid materials. However, the use of solid working electrodes brings forth several problems. In many cases chemical changes of the electrode material occur on the electrode surface and a large amount of hydrogen is created in comparison to a mercury electrode, causing interfering background current.
The influence of ultrasound on an electrode and on the electrolyte surrounding it using different measurement methods is described for instance in Soviet Union Pat. No. 219,860. The said Soviet Union patent employs an ultrasound field in a polarographic measuring method, and the purpose of the ultrasonic field is to keep the electrode clean and to improve measuring sensitivity owing to the advantageous agitating effect caused by the field. Ultrasound is also used in an amperometric measuring method, U.S. Pat. No. 4,033,830, for keeping the necessary counter- and measuring electrodes clean. As a conclusion, the aforementioned patents use ultrasound only for improving the sensitivity of the electrodes and for keeping the electrodes clean. When employed in this fashion, however, ultrasound does not essentially improve the conditions of the analysis carried out by solid electrodes.
The purpose of the present invention is to eliminate some of the drawbacks in the prior art and to achieve a new and better method for the voltammetric measurement of the component contents of a liquid phase by utilizing ultrasound during the measurement.